The 21-year-old suffered a significant leg wound in a late-night shark attack while wading in knee-deep water in Riverton about 2am on Monday.
The mother said her daughter "has a long recovery ahead of her".
"She is in ICU under the care of my brother and she is doing well after surgery," she said.
The family was flying to Invercargill to be with the victim at Southland Hospital.
"We are from Riverton, so this will hit the community hard as this is an area that many people take their children to." The mother was in London when she heard the news and booked a flight home as soon as possible, she said.
Police said she received a significant laceration to her leg that required treatment from emergency services.
"The culprit is possibly a sevengill shark, which is present in the estuary.
"The time of day is a possible factor."
Department of Conservation marine scientist Clinton Duffy said sevengill sharks were common along the Southland and Otago coastlines at this time of year.
"Like many shark species, [they] forage actively at night.
"Sevengill sharks in particular are known to come into shallow water areas," Mr Duffy said.
About one to two bites are recorded in New Zealand yearly.
"Sevengill sharks are the most common reported.
"They are thought to be the species responsible for most of the reported bites in the Southland area over recent years."
A Te Whatu Ora Southern spokeswoman said the woman was in a stable condition.
Hato Hone St John confirmed they were notified at 2.27am on Monday "of an incident involving an animal".
"We responded [with] one ambulance and one rapid-response unit to Leader St, River-ton," a spokesman said.
"Our paramedics treated and transported one patient in a serious condition to Southland Hospital."
Additional reporting Ben Andrews
